Researchers at the University of Illinois at Urbana-Champaign have utilized a series of simulations to model the performance of twin-engine hybrid-electric general-aviation propulsion systems. Their paper appears in the Journal of Aircraft.

They created a flight-performance simulator to represent accurately the true flight performance of a Tecnam P2006T on a general mission to include take off, climb, cruise, descent, and landing, along with sufficient reserves to meet FAA regulations. Transition segments were incorporated into the simulation during climb and descent where the throttle setting, flap deployment, propeller rotation rate, and all other flight control variables were either set to mimic input from a typical pilot or prescribed in accordance with the aircraft flight manual.

After configuring the simulator to collect baseline performance data, a parallel hybrid drivetrain was integrated into the simulation. The researchers compared the sensitivity of range and fuel economy to the level of electrification, battery specific energy density, and electric motor power density. The same sensitivities were studied with a series hybrid-electric drivetrain.

a) parallel and b) series drivetrain models

They found that current technology allows a parallel hybrid configuration to achieve a maximum theoretical range of approximately 175 n mile. The results also indicated that parallel hybrid architectures will offer an effective near-term configuration, by offering greater range performance than a series hybrid with incremental future advancements in battery specific energy density and electric motor power density.

However, distant future advancements in these technologies will allow series-hybrid architectures to produce similar range capabilities with improved fuel economy over parallel-hybrid architectures.

Jet fuel and aviation gasoline are easy to store on an airplane. They are compact and lightweight when compared to the amount of energy they provide. Unfortunately, the actual combustion process is very inefficient. We’re harnessing only a small fraction of that energy but we currently don’t have electrical storage systems that can compete with that.

—Phillip Ansell, assistant professor in the Department of Aerospace Engineering in the College of Engineering at the University of Illinois

Ansell said adding more batteries to fly farther may seem logical, but it works against the goal to make an aircraft as lightweight as possible.

Ansell said that, overall, a hybrid-electric drivetrain can lead to substantial improvements in fuel efficiency of a given aircraft configuration, though these gains depend strongly on the coupled variations in the degree of drivetrain electrification and the required mission range. Both of these factors influence the weight allocation of battery and fuel systems, as well as the weight scaling imposed by internal combustion engine and electrical motor components. In general, to obtain the greatest fuel efficiency a hybrid architecture should be used with as much electrification in the drivetrain as is permissible within a given range requirement.

The fuel efficiency improvements were shown to particularly shine for short-range missions, which is a good thing since range limitations serve as one of the key bottlenecks in hybrid aircraft feasibility.

One interesting and unexpected result we observed, however, came about when comparing the parallel and series hybrid architectures. Since the parallel architecture mechanically couples the shaft power of the engine and motor together, only one electrical machine is needed. For the series architecture, a generator is also needed to convert the engine power to electrical power, along with a larger motor than the parallel hybrid configuration to drive the propulsor. Unexpectedly, this aspect made the parallel architecture more beneficial for improved range and fuel burn almost across the board due to its lighter weight. However, we did observe that if significant improvements are made in maturing electrical motor components in the very long term, we may actually someday see better efficiency out of series-hybrid architectures, as they permit a greater flexibility in the placement and distribution of propulsors.

—Phillip Ansell

This project was supported by NASA Neil A. Armstrong Flight Research Center under Small Business Technology Transfer in collaboration with Rolling Hills Research Corporation.